Measuring devices of process and automation technology most often include: at least one probe unit, which, based on the process variable or a change of the process variable, produce a measured variable; and an end stage. The end stage produces a suitable output signal as a function of the measured variable, or the process variable, or a change of the two variables. In the case of many electrical current controlled, end stages, which produce e.g. a 4 . . . 20 mA signal, the function of an interlock diode is offered. Therewith, operating personnel, or customers, can measure directly at the measuring device, with an ordinary multimeter, the actual value of the electrical current output; i.e. the measured value can be tapped directly on-site. A disadvantage of such an arrangement is that, when the interlock diode is not shunted by a multimeter measuring device, i.e. no measuring is being performed, the interlock diode is switched into the 4 . . . 20 mA, electrical current path. There is, then, always a certain voltage drop, e.g. 0.7 V, across the diode. The circuit parts connected downstream lack this voltage in their supply. This leads, in the case of devices operated with low terminal voltage, often to problems, since the measuring devices must function just with the remaining voltage (e.g. 9.3 V instead of a 10 V terminal voltage).